cleanup(pubsub): remove internal builders from public API

[haphungw]

Use internal_builders config in librarian.yaml to generate pub(crate) visibility for pubsub dataplane builders. This also removes the previous #[doc(hidden)] workaround used to avoid doctest failures.

Fix #3959

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (14367c9) to head (1832807).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4524   +/-   ##
=======================================
  Coverage   95.01%   95.01%           
=======================================
  Files         195      195           
  Lines        7449     7449           
=======================================
  Hits         7078     7078           
  Misses        371      371           

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[PhongChuong]

The changes LGTM but you may need to update the librarian version/regenerate the rest of the repo prior to this PR. It might be useful to check with the current on duty.

[dbolduc]

Nice.